GENUS B ALAN US. 183 



parietes is absent. In B.flosculus the basis is calcareous, 

 but consists of so excessively thin a film as hardly to be 

 distinguished : it presents, moreover, as also is the case 

 with B. imperator, a beaded structure. Again, in some 

 few species, as in B. balanoides, the basis is simply mem- 

 branous. When the basis is thin, it is always flat, and 

 is closely moulded to the irregular surface of attachment • 

 and in this case, when specimens are crowded together, 

 their elongation is effected exclusively by the growth of 

 the walls ; but, when the basis is thick, it sometimes be- 

 comes, in crowded groups, deeply, but irregularly, cup- 

 formed, or cylindrical, as in B. psittacus and perforatus. 

 In B. allium, however, which inhabits massive corals, the 

 basis is as regularly concave or cup-formed as in the genus 

 Pyrgoma : in B. calceolus and its allies, and in some varieties 

 of the fossil B. inclusus, the basis is boat-formed, with its 

 lower surface deeply grooved longitudinally from clasping 

 the stem of the Gorgonia or other zoophyte, to which it 

 was attached. In certain varieties of B. Icevis it is very 

 remarkable that the deeply cup-formed basis becomes, 

 owing apparently to the whole shell having grown too deep 

 for the animal, half-filled up with irregular, calcareous, 

 transverse plates (PI. 4, fig. 2 a), resting one upon 

 the other by irregular points or pillars. The cement- 

 ing apparatus has been sufficiently described under the 

 Family. 



Mouth. — The labrum is always notched; sometimes it 

 has no teeth, but generally there are three on each side ; 

 in B. balanoides there are five or six on each side ; and in 

 B. improvisus and eburneus there is a whole row of teeth 

 (PL 26, fig. 2, e), graduated in size, on each side of the 

 notch. The palpi are large, with their apices nearly touch- 

 ing, and furnished with long spines. The mandibles have, 

 as it appears, normally, five teeth, but the two lower teeth 

 are always small and often rudimentary, and almost con- 

 fluent with the inferior, sometimes spinose angle. The 

 maxilla have either a simple edge, or a notch under the 

 pair of large upper spines, or the lower part forms (PL 26, 

 i\g. 7) a step-formed projection : there are generally two 

 lower spines, placed singly or not in pairs, larger than the 



